SiC power devices have been intensively investigated for the past 13 years. High power SiC vertical junction field effect-transistors (VJFETs) have attracted great attention for high temperature applications because VJFETs do not suffer from the low channel mobility problem of SiC MOSFETs. One SiC VJFET attempt, U.S. Pat. No. 6,107,649 to J. H. Zhao entitled Field-controlled high power semiconductor devices, the disclosure of which is hereby incorporated as reference, solves the problem of high electric field in the gate oxide of SiC MOSFETs by using lateral FETs to control the conduction of vertical channels without the need of epitaxial regrowth. FIG. 1 is a copy of FIG. 6A from U.S. Pat. No. 6,107,649. Another attempt, as found in the paper by K. Asano et al. entitled 5 kV 4H—SiC SEJFET with low RonS of 69 mΩcm2 published in IEEE ISPSD-2002, pp. 61-64, cited herein as reference, has described a normally-off VJFET as shown in FIG. 2 which also uses a lateral JFET to control a vertical channel but requires expensive epitaxial regrowth at the middle of the device fabrication. The use of lateral JFET clearly results in higher device resistance leading to low current capability.
Purely vertical JFETs without the lateral JFETs have also been attempted but mostly in the forms of static induction transistors (SITs) which do not have long and highly uniform opening vertical channels defined and controlled by vertical pn junction gates. One attempt, as shown in FIG. 3, FIG. 4 and FIG. 5 which are copies of FIG. 5, FIG. 3 and FIG. 10, respectively, from U.S. Pat. No. 5,903,020 to R. R. Siergiej et. al. entitled Silicon Carbide static induction transistor structure, cited herein as reference, describes the formation of the p+ gates by normal incident, planar ion implantation on planar surface as shown herein in FIG. 3, by normal incident, planar ion implantation onto shallowly etched surface as shown herein in FIG. 4, and by normal incident, planar ion implantation onto only the deep trench bottoms as shown herein in FIG. 5. These planar, normal incident ion implantation approaches do not result in long vertical channels with uniform channel opening dimensions as stated in U.S. Pat. No. 5,903,020. Without highly uniform opening and long vertical channel, these SITs can not support high voltages. Besides they are difficult to be made normally-off switches capable of high voltage and high current. FIG. 6, a copy of FIG. 1 in the paper by J. Nishizawa et al. entitled The 2.45 GHz 36 W CW Si recessed gate type SIT with high gain and high voltage operation in IEEE Transactions on Electron Devices, Vol. 4, No. 2, February 2000, pp. 482-487, cited herein as reference, shows the well known silicon-based SIT design similarly without long vertical channels of a highly uniform channel opening dimension defined and controlled by p+n junctions. One attempt has been reported to develop purely vertical JFETs with long vertical channels up to 2 um by Mega-eV ion implantation but without a highly uniform channel opening dimension, as shown in FIG. 7 which is a copy of FIG. 1 in the paper by H. Onose, et al. entitled 2 kV 4H—SiC junction FETs published by Materials Science Forum, Vols. 389-393, 2002, pp. 1227-1230, cited herein as reference. The long vertical channel defined by Xj=2 um shown herein as FIG. 7 has a curved channel with a highly non-uniform vertical channel opening and a minimum opening dimension of Wch. In fact, SIT and VJFET gates formed by normal incident ion implantation generally lead to highly non-uniform channel opening dimensions similar to the curved channel of FIG. 7. Although the channel is near 2 um, the vertical part of the channel that has the same channel opening dimension is negligible in comparison to the total channel length. Hence, with a gate to source reverse bias as high as 50V to shut off the channel and create a large enough source-to-drain barrier, the vertical JFET blocks only 2 kV, much below the theoretical blocking voltage limit of >3 kV for the 20 um thick structure shown in FIG. 7 because of the absence of a long vertical channel with a highly uniform channel opening dimension. Besides, the device specific on-resistance is as high as 70 mΩcm2. Furthermore, the vertical JFET is a normally-on switch, again due to the absence of long vertical channel with a highly uniform opening dimension defined and controlled to be normally-off by highly vertical p+n junctions. In fact, to the best of the inventor's knowledge, no normally-off VJFETs have been reported without using the present invention, although high power control systems clearly need normally-off switches to provide the important fail-safe protection. It would be clear to those skilled in the art that, to achieve a low device resistance in the conduction mode and to block high voltage in the blocking mode, a high voltage VJFET requires a large depth of the barrier between the drain and source to prevent the well know drain-induced barrier lowering (DIBL) in FETs which leads to high leakage current and early breakdown of the switch. When the potential barrier along the source to drain direction is short in barrier depth as is the case for the vertical JFET shown in FIG. 7 and SITs shown in FIGS. 3, 4, 5, and 6 as well as any other VJFETs and SITs with vertical channel defined by normal incident ion implantation gate, a large enough drain (blocking) voltage will pull down the barrier, causing electrons to flow from the source over the reduced barrier to the drain resulting in high leakage and early breakdown. Hence, up to date, all reported pure VJFETs and SITs without using present invention are normally-on and require a large negative gate voltage to over-pinch and shut off the channel so that a large enough source-to-drain barrier depth can be created for the devices to block high voltages. But gate-to-source pn junctions heavily doped on both sides tend to have a lower breakdown voltage and a larger leakage current under high reverse gate bias, not desirable for high power transistors.
Therefore, it is obvious to those skilled in the art that pure vertical JFETs with gate junctions formed by normal incident ion implantation without long vertical channels of a highly uniform channel opening dimension are not desirable for the implementation of normally-off operation. It is also obvious to those skilled in the art that pure vertical JFETs can not offer optimum normally-on operation when the gate junctions are formed by normal incident ion implantation without long vertical channels of a highly uniform channel opening dimension because an excessive negative gate bias is needed to create a barrier with enough depth to block desired voltages.
There is, therefore, a clear need to design a better performing SiC VJFET with long vertical channels all having a highly uniform channel opening dimension defined and controlled by highly vertical gate p+n junctions so that higher power capability can be achieved with lower device resistance for either normally-off or normally-on operation.